Antennas are used as coupling elements between circuits (e.g., radio frequency (RF) circuits) and a wireless medium. Antenna design usually uses metallic conductors to transfer energy between electromagnetic waves and electrical currents. An antenna's size is usually related to a target wavelength. For example, an antenna may have a characteristic dimension (e.g., length) that is ½ or ¼ of a target wavelength.
Some antennas may be used to operate over a wide operating frequency band. Fixed tuning mechanisms, such as frequency-selective lumped filters (called “traps”), may be used to create multi-band antennas that selectively add reactance to portions of a longer antenna to allow the antenna to operate in frequency bands with shorter wavelengths. Reactance may be added to the portions of the antenna by joining segments of a metallic radiating structure of the antenna using traps or filters. Multi-band antennas may also be created using distributed resonant (LC) element structures.
When a spectral coverage of an antenna is very large, it may be useful to have an antenna that is “active” in the sense that metallic element segments of the antenna may be adjusted remotely. One approach has been to include an extendable telescoping mast in the antenna. A length of the telescoping mast may be varied by using a motor drive system. Using an extendable telescoping mast for an antenna in the microwave region may be impractical, as the antenna may be small and the adjustments may distort the behavior of the antenna.
Another approach to adjust an antenna such that a radiating length is matched to a frequency of interest is to use electrical switches or relays. Control and power circuits used to provide adjustment may disrupt the radiative behavior of the antenna due to parasitic conductors that behave as unwanted radiating elements.